1. Field of the Invention
The present invention related to a liquid crystal display device, and more particularly, to a liquid crystal display capable of reducing power consumption by charge sharing.
2. Description of the Prior Art
Due to advantages such as low radiation, thin appearance and low power consumption, liquid crystal display (LCD) devices have gradually replaced traditional cathode ray tube (CRT) displays and been widely used in notebook computers, personal digital assistants (PDA), flat panel televisions or mobile phones.
Reference is made to FIG. 1 for a diagram of a prior art LCD device 10. The LCD device 10 includes an LCD panel 12, a timing controller 14, a source driver 16, and a gate driver 18. The LCD panel 12 includes a plurality of parallel data lines D1-Dm, a plurality of parallel gate lines G1-Gn, and a plurality of display units P11-Pmn. The data lines D1-Dm intersect the gate lines G1-Gn, and each of the display units P11-Pmn is disposed at the intersection of a corresponding data line and a corresponding gate line. The timing controller 14 can generate data signals corresponding display images, as well as control signals and clock signals for driving the LCD panel 12. Based on signals received from the timing controller 14, the source driver 16 and the gate driver 18 generate corresponding gate signals and driving signals, respectively. Each display unit of the LCD panel 12 includes a thin film transistor (TFT) switch and an equivalent capacitor. Each equivalent capacitor has an end coupled to a corresponding data line via a corresponding TFT switch, and another end coupled to a common voltage Vcom (Cs on common). When the TFT switch of a display unit is turned on by a gate signal generated by the gate driver 18, the equivalent capacitor of the display unit is electrically connected to its corresponding data line and can thus receive a driving voltage from the source driver 16. Therefore, the display unit can display images of various gray scales by changing the rotation of liquid crystal molecules based on charges stored in the equivalent capacitor.
With increasing demands in large-size applications, the panel loading and dynamic power consumption also increase as the LCD panel becomes larger. As a result, it is a main concern to lower power consumption when designing an LCD device. Generally speaking, in order to avoid permanent polarization of liquid crystal materials, the polarities of voltages applied to both ends of equivalent capacitors have to be reversed periodically. Common methods for driving LCD panels include dot inversion and line inversion. When the driving voltages of an LCD device begin to reverse respective polarities, the LCD device has the largest loading since the source driver consumes the largest amount of current at this point of time.
Charge sharing is normally applied for reducing power consumption in an LCD device. Charge sharing halves the amount of dynamic current by rearranging charges before the source driver outputs driving signals. In the prior art LCD device 10, the source driver 16 includes a plurality of output buffers 22 and a plurality of charge sharing switches 24. The source driver 16 can output driving signals to corresponding data lines via the output buffers 22. The charge sharing switches 24, each coupled between two neighboring data lines, are used for performing charge sharing. Assuming dot-inversion is used for driving the LCD panel 12 of the LCD device 10, among the driving voltages outputted by the source driver 16 to the data lines D1-Dm, half of them are higher than the common voltage Vcom, while the other half are lower than the common voltage Vcom. In other words, during positive driving periods, the source driver 16 outputs a driving voltage VPIXEL—POSITIVE higher than the common voltage Vcom to odd-numbered date lines D1-Dm-1, and outputs a driving voltage VPIXEL—NEGATIVE lower than the common voltage Vcom to even-numbered date lines D2-Dm; during negative driving periods, the source driver 16 outputs the driving voltage VPIXEL—NEGATIVE to odd-numbered date lines D1-Dm-1, and outputs the driving voltage VPIXEL—POSITIVE to even-numbered date lines D2-Dm. The values of the driving voltages VPIXEL—POSITIVE and VPIXEL—NEGATIVE depend on the gray scales of display images,
Before outputting the driving voltages, the prior art LCD device 10 turns on the charge sharing switches 24 in order to neutralize residual charges stored in the data lines at the end of previous driving periods. Reference is made to FIG. 2 for a diagram illustrating the voltage level of a liquid crystal capacitor in the LCD device 10. In FIG. 2, the transverse axle represents time, the vertical axle represents voltage level, VMAX and VMIN respectively represent the maximum and the minimum driving voltages outputted to the equivalent capacitor, and VAVG represents the voltage level of each data line after charge sharing. During the positive driving period, the driving voltage VPIXEL—POSITIVE outputted to the liquid crystal capacitor is between the common voltage Vcom and the maximum driving voltage VMAX; during the negative driving period, the driving voltage VPIXEL—NEGATIVE outputted to the equivalent capacitor is between the common voltage Vcom and the minimum driving voltage VMIN.
Assuming dot-inversion is used for driving the LCD panel 12 of the LCD device 10, the display units P11 and P12 are used for illustrations. In FIG. 2, the equivalent capacitor of the display unit P11 has a voltage level VPIXEL—NEGATIVE equal to the minimum driving voltage VMIN and the equivalent capacitor of the display unit P12 has a voltage level VPIXEL—POSITIVE equal to the maximum driving voltage VMAX at the end of the previous negative driving period (at T1). Before outputting driving voltages to the display unit P11 during the current positive driving period (between T1 and T2), the prior art LCD device 10 turns on the charge sharing switch 24 coupled between the data lines D1 and D2 in order to neutralize residual charges stored in the corresponding data line at the end of the previous negative driving period. Therefore, the voltage level of the equivalent capacitor in the display unit P11 is raised from VPIXEL—NEGATIVE to VAVG. When VPIXEL—POSITIVE and VPIXEL—NEGATIVE are respectively equal to the maximum driving voltage VMAX and the minimum driving voltage VMIN, VAVG is equal to the common voltage Vcom. During the current positive driving period, the prior art LCD device 10 only needs to provide a voltage difference ΔVp to the display unit P11. The value of ΔVp depends on the gray scale of images to be displayed by the display unit P11, and can be represented by the following formula:0≦ΔVp=(VPIXEL—POSITIVE−VAVG)≦(VMAX+VMIN)/2
Similarly, the equivalent capacitor of the display unit P11 has a voltage level VPIXEL—POSITIVE equal to the maximum driving voltage VMAX and the liquid crystal capacitor of the display unit P12 has a voltage level VPIXEL—NEGATIVE equal to the minimum driving voltage VMIN at the end of the previous positive driving period (at T2). Before outputting driving voltages to the display unit P11 during the current negative driving period (between T2 and T3), the prior art LCD device 10 turns on the charge sharing switch 24 coupled between the data lines D1 and D2 in order to neutralize residual charges stored in the corresponding data line at the end of the previous positive driving period. Therefore, the voltage level of the equivalent capacitor in the display unit P11 is decreased from VPIXEL—POSITIVE to VAVG. When VPIXEL—POSITIVE and VPIXEL—NEGATIVE are respectively equal to the maximum driving voltage VMAX and the minimum driving voltage VMIN, VAVG is equal to the common voltage Vcom. During the current negative driving period, the prior art LCD device 10 only needs to provide a voltage difference ΔVn to the display unit P11. The value of ΔVn depends on the gray scale of images to be displayed by the display unit P11, and can be represented by the following formula:0≦ΔVn=(VAVG−VPIXEL—NEGATIVE)≦(VMAX+VMIN)/2
Without charge sharing, the prior art LCD device 10 needs to provide a voltage difference ΔV to a display unit. The value of ΔV can be represented by the following formula:0≦|ΔV|≦(VMAX+VMIN)Therefore,ΔVp≦|ΔV| and ΔVn≦|ΔV|
The prior art LCD device 10 uses the charge sharing switches 24 for performing charge sharing. The power consumption can be reduced since the LCD device 10 only needs to provide display units with the voltage differences ΔVp and ΔVn, whose absolute values are smaller than that of the voltage difference ΔV. However, the charge sharing switches 24 are disposed on the source driver 16. Since a large number of charge sharing switches 24 are required in large-size applications and generate a lot of heat during charge sharing, the source driver 16 can encounter difficulties in heat dissipation.